media: i2c: st-vgxy61: Use VGXY61_NB_POLARITIES instead of hardcoded value in tx_from_ep

tx_from_ep's for loop uses '5' as bound, while in fact it refers to the
number of polarities. Replace it by VGXY61_NB_POLARITIES for
factorization.

Signed-off-by: Benjamin Mugnier <benjamin.mugnier@foss.st.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>