[media] stb0899: restore minimal rate to 5Mbauds

According with Manu Abraham, stb0899 seek algorithm is broken
for symbol rates bellow to 5Mbauds. So, revert those patches:

55b3318b [media] stb0899: allow minimum symbol rate of 2000000
2eeed77f

 [media] stb0899: allow minimum symbol rate of 1000000

Requested-by: Manu Abraham <abraham.manu@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>