file2alias: Support for serdev devices

This patch allows file2alias to generate the proper module headers to
support serdev modalias drivers.

Eg:

root@qt5022:~# modinfo serdev:ttydev | grep alias
alias:          serdev:ttydev_serdev
alias:          serdev:ttydev

root@qt5022:~#
	cat /lib/modules/4.16.0-qtec-standard/modules.alias | grep serdev
alias serdev:ttydev_serdev serdev_ttydev
alias serdev:ttydev serdev_ttydev

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>