USB: gadget: u_bam: Fix list corruption issue

Currently USB driver is allocating usb requests without holding
spinlock, but deallocating usb requests with spinlock holding.
This results in list corruption if allocation and deallocation
of usb requests race each other during fast plugin/plugout
scenarios. Hence take spinlock before allocation of USB requests
to fix the issue.

CRs-Fixed: 686006
Change-Id: I827d9fa7fa6ef975f4e1d530268b08daa89f1bc1
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>