media: cec: fix smatch error

drivers/media/cec/cec-pin-error-inj.c:231
cec_pin_error_inj_parse_line() error: uninitialized symbol 'pos'.

The tx-add-bytes command didn't check for the presence of an argument, and
also didn't check that it was > 0.

This should fix this error.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>