[media] stv0900_core: don't allocate a temporary var

The error return code STV0900_NO_ERROR happens only once, at
the end of the functions. So, just return it directly.

This driver should actually be fixed to return standard
Linux error codes, instead of its own macros, but this
should be done on a separate patchset.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>