[media] gspca: avoid unused variable warnings

When CONFIG_INPUT is disabled, multiple gspca backend drivers
print compile-time warnings about unused variables:

media/usb/gspca/cpia1.c: In function 'sd_stopN':
media/usb/gspca/cpia1.c:1627:13: error: unused variable 'sd' [-Werror=unused-variable]
media/usb/gspca/konica.c: In function 'sd_stopN':
media/usb/gspca/konica.c:246:13: error: unused variable 'sd' [-Werror=unused-variable]

This annotates the variables as __maybe_unused, to let the compiler
know that they are declared intentionally.

Fixes: ee186fd9 ("[media] gscpa_t613: Add support for the camera button")
Fixes: c2f644ae ("[media] gspca_cpia1: Add support for button")
Fixes: b517af72 ("V4L/DVB: gspca_konica: New gspca subdriver for konica chipset using cams")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>