[media] staging: as102: Remove linkage specifiers for C++

The as10x_cmd.h header is not public so there should be no need
for an "extern "C"" in it.

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