USB: serial: use IS_ENABLED() instead of checking for built-in or module

The IS_ENABLED() macro checks if a Kconfig symbol has been enabled either
built-in or as a module, use that macro instead of open coding the same.

Using the macro makes the code more readable by helping abstract away some
of the Kconfig built-in and module enable details.
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/keyspan_pda.c

@@ -30,12 +30,12 @@
 #include <linux/usb/ezusb.h>
 
 /* make a simple define to handle if we are compiling keyspan_pda or xircom support */
-#if defined(CONFIG_USB_SERIAL_KEYSPAN_PDA) || defined(CONFIG_USB_SERIAL_KEYSPAN_PDA_MODULE)
+#if IS_ENABLED(CONFIG_USB_SERIAL_KEYSPAN_PDA)
 	#define KEYSPAN
 #else
 	#undef KEYSPAN
 #endif
-#if defined(CONFIG_USB_SERIAL_XIRCOM) || defined(CONFIG_USB_SERIAL_XIRCOM_MODULE)
+#if IS_ENABLED(CONFIG_USB_SERIAL_XIRCOM)
 	#define XIRCOM
 #else
 	#undef XIRCOM