tpm: Rename tpm.c to tpm-interface.c

This is preparation for making the tpm module multi-file. kbuild does
not like having a .c file with the same name as a module. We wish to
keep the tpm module name so that userspace doesn't see this change.

tpm-interface.c is chosen because the next several commits in the series
migrate items into tpm-sysfs.c, tpm-dev.c and tpm-class.c. All that will
be left is tpm command processing and interfacing code.
Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>

drivers/char/tpm/Makefile

@@ -2,6 +2,8 @@
 # Makefile for the kernel tpm device drivers.
 #
 obj-$(CONFIG_TCG_TPM) += tpm.o
+tpm-y := tpm-interface.o
+
 ifdef CONFIG_ACPI
 	obj-$(CONFIG_TCG_TPM) += tpm_bios.o
 	tpm_bios-objs += tpm_eventlog.o tpm_acpi.o tpm_ppi.o