media: v4l: cadence: Fix how unsued lanes are handled in 'csi2rx_start()'

The 2nd parameter of 'find_first_zero_bit()' is a number of bits, not of
bytes. So use 'csi2rx->max_lanes' instead of 'sizeof(lanes_used)'.

Fixes: 1fc3b37f ("media: v4l: cadence: Add Cadence MIPI-CSI2 RX driver")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>