USB: gadget: u_bam: Fix spinlock lockup issue

Release spinlock before schedueling work when bam channel
is ready after cable is connected. Otherwise it leads
to spinlock lockup issue.

Also release spin lock before usb requests allocation to fix
"BUG: calling sleep function in atomic context".

Change-Id: Ia7f0bb0cf0096e30488e55a93cf62a2d8644a818
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>