radeon/i2c: do not count reg index in number of i2c byte we are writing.

Useless to count the register index in number of bytes we are writing.

Fixes a regression with hw i2c enabled.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org