usb: gadget: f_mtp: simplify ptp NULL pointer check

Simplify MTP/PTP dev NULL pointer check introduced in
Change-Id: Ic44a699d96df2e13467fc081bff88b97dcc5afb2
and restrict it to MTP/PTP function level only.

Return ERR_PTR() instead of NULL from mtp_ptp function
to skip doing NULL pointer checks all the way up to
configfs.c

Fixes: Change-Id: Ic44a699d96df2e13467fc081bff88b97dcc5afb2
       ("usb: gadget: fix NULL ptr derefer while symlinking PTP func")
Change-Id: Iab7c55089c115550c3506f6cca960a07ae52713d
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>