media: frontends/stv0910: add field offsets to field defines

Each field (FSTV0910_XX) is declared as reg/off/unused/sign/mask. Add
the missing offset value to the defines.

Picked up from dddvb master, commit 8a1f27c3d22c ("add field offset to
field defines") by Ralph Metzler <rjkm@metzlerbros.de>, adapted to
match the comment style in the mainline driver.

Cc: Ralph Metzler <rjkm@metzlerbros.de>
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>