[media] stb0899: set FE_HAS_SIGNAL flag in read_status

In stb0899_read_status the FE_HAS_SIGNAL flag was not set in case of a
successful carrier lock. This change fixes that.

Signed-off-by: Andreas Regel <andreas.regel@gmx.de>
Acked-by: Klaus Schmidinger <Klaus.Schmidinger@tvdr.de>
Cc: Manu Abraham <abraham.manu@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>