[media] c8sectpfe: Rework firmware loading mechanism

c8sectpfe driver relied on CONFIG_FW_LOADER_USER_HELPER_FALLBACK option
for loading its xp70 firmware. A previous commit removed this Kconfig
option, as it is apparently harmful, but did not update the driver
code which relied on it.

This patch reworks the firmware loading into the start_feed callback.
At this point we can be sure the rootfs is present, thereby removing
the depedency on CONFIG_FW_LOADER_USER_HELPER_FALLBACK.

Fixes: 79f5b6ae960d ('[media] c8sectpfe: Remove select on CONFIG_FW_LOADER_USER_HELPER_FALLBACK')

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>