usb: gadget: mbim: Release lock while copying from userspace

Reduce time spent in critical section by performing
memory allocation and copy operations before
the spin_lock is taken. This improves MBIM performance
by increasing overall throughput from 100 KB/s to 3 MB/s.

Change-Id: I16933633c0d2794d4cdad1a78e693f630a2e26f5
Signed-off-by: Anna Perel <aperel@codeaurora.org>