V4L/DVB: dib0090: cleanup dib0090_dcc_freq()

'extern' is not needed at function definition.

This will remove the following sparse warning (see "make C=1"):
 * function 'dib0090_dcc_freq' with external linkage has definition

Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>