[media] dib7000p: avoid division by zero

[media] dib7000p: avoid division by zero

dib7000p_read_word() may return zero on i2c errors, resulting in
dib7000p_get_internal_freq() returning zero.
So don't divide by the result of dib7000p_get_internal_freq()
without checking it for zero in dib7000p_set_dds().

On one of my machines the device
ID 2304:0229 Pinnacle Systems, Inc. PCTV Dual DVB-T 2001e
about once a day/every two days gets into a state, where
most (all?) I2C reads return with an error. Tuning during this
state will result in a divide by zero without this patch.
This patch doesn't fix the root cause for the device getting
into a bad state, but it allows me to unload/reload the drivers,
bringing it back into a usable state.

Signed-off-by: Martin Wache <M.Wache@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>