usb: gadget: f_mtp:Fix issue of NULL pointer access in mtp_read

MTP usb device node created as a part of mtp function init call.
Userspace can read/write to MTP device using this node. If MTP is
not enabled in the composition and trying to read mtp_usb dev node
from the userspace leading to null pointer access in mtp_read.

Do not access ep OUT maxpacket size in mtp_read. First block on mtp_read
until the state become online which doesn't wakeup from the thread and
expecting for the read completion or state change which occurs as
a part of set_alt.

Change-Id: Icbee5fe7ae2c02b2bca185a0dc7587eb4940058a
Signed-off-by: ChandanaKishori Chiluveru <cchilu@codeaurora.org>