Some functions are only used locally so mark them static.

This will remove the following sparse warnings (see "make C=1"):
 * symbol 'extract_mask_pos' was not declared. Should it be static?
 * symbol 'stv0900_initialize' was not declared. Should it be static?
 * symbol 'stv0900_get_mclk_freq' was not declared. Should it be static?
 * symbol 'stv0900_set_mclk' was not declared. Should it be static?
 * symbol 'stv0900_get_err_count' was not declared. Should it be static?
Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>