thinkpad-acpi: enable loading module with new B-series Lenovo BIOS

The new B series BIOS has version string 43CN46WW. The driver
requires that 2nd and 3rd characters be 'E' and 'T' respectively,
where as the newer BIOS has 'C' and 'N' respectively. Failing to
load the module causes some of the hotkeys to not work.

Before the patch
================
sudo modprobe thinkpad_acpi
FATAL: Error inserting thinkpad_acpi (/lib/modules/3.5.0-15-generic/kernel/drivers/platform/x86/thinkpad_acpi.ko): No such device

After the patch
===============
[44937.265438] thinkpad_acpi: ThinkPad ACPI Extras v0.24
[44937.265445] thinkpad_acpi: http://ibm-acpi.sf.net/
[44937.265449] thinkpad_acpi: ThinkPad BIOS 43CN46WW, EC unknown
[44937.265453] thinkpad_acpi: Lenovo Lenovo B470e, model HuronRiver Platform
[44937.266479] thinkpad_acpi: detected a 8-level brightness capable ThinkPad
[44937.266557] thinkpad_acpi: Standard ACPI backlight interface available, not loading native one
[44937.267846] thinkpad_acpi: Console audio control enabled, mode: monitor (read only)
[44937.268131] input: ThinkPad Extra Buttons as /devices/platform/thinkpad_acpi/input/input17
Signed-off-by: Manoj Iyer <manoj.iyer@canonical.com>
Tested-by: James Ferguson <james.ferguson@canonical.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>

drivers/platform/x86/thinkpad_acpi.c

@@ -8574,7 +8574,8 @@ static bool __pure __init tpacpi_is_valid_fw_id(const char* const s,
 	return s && strlen(s) >= 8 &&
 		tpacpi_is_fw_digit(s[0]) &&
 		tpacpi_is_fw_digit(s[1]) &&
-		s[2] == t && s[3] == 'T' &&
+		s[2] == t &&
+		(s[3] == 'T' || s[3] == 'N') &&
 		tpacpi_is_fw_digit(s[4]) &&
 		tpacpi_is_fw_digit(s[5]);
 }
@@ -8607,7 +8608,8 @@ static int __must_check __init get_thinkpad_model_data(
 		return -ENOMEM;
 
 	/* Really ancient ThinkPad 240X will fail this, which is fine */
-	if (!tpacpi_is_valid_fw_id(tp->bios_version_str, 'E'))
+	if (!(tpacpi_is_valid_fw_id(tp->bios_version_str, 'E') ||
+	      tpacpi_is_valid_fw_id(tp->bios_version_str, 'C')))
 		return 0;
 
 	tp->bios_model = tp->bios_version_str[0]