firmware: google: fix gsmi.c build warning

Use min_t() macro instead of min() to fix a build warning:

  CC      drivers/firmware/google/gsmi.o
drivers/firmware/google/gsmi.c: In function ‘gsmi_get_variable’:
drivers/firmware/google/gsmi.c:348: warning: comparison of distinct
pointer types lacks a cast

Signed-off-by: Maxin B. John <maxin.john@gmail.com>
Acked-By: Mike Waychison <mikew@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>