[media] staging: as102: Move variable declarations to the header

Fixes following checkpatch.pl warning:
WARNING: externs should be avoided in .c files

Cc: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Sylwester Nawrocki <snjw23@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>