V4L/DVB (13292): tvp514x: recognize the error case in tvp514x_read_reg()

i2c_smbus_read_byte_data() returns a negative value on error. It is very
likely to be != -1 (-EPERM).

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>