drm/radeon: radeon_dp_i2c_aux_ch() must return 0 on FreeBSD

The code was unmodified compared to Linux and returned the amount of
received bytes from the i2c bus. This led to non-working i2c bus and
failure to eg. read monitor's EDID, if connected to DisplayPort.

MFC after:	3 days
Tested by:	Mikaël Urankar <mikael.urankar@gmail.com>