usb: gadget: f_mtp: Optimize usage of read_mutex

Driver currently acquires and releases read_mutex multiple
times during single file transfer operation resulting in
throughput degradation. Fix this by holding mutex for
longer times without releasing it as it is perfectly fine
to call usb_ep_queue() while holding a mutex.

Change-Id: Ic235fb92997993bd75ed5e519a5d73e787522810
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>
Signed-off-by: Udipto Goswami <ugoswami@codeaurora.org>