USB: gadget: u_bam: Free usb requests during disconnect

Currently USB requests are allocated during connect work for starting
endless TX and RX in BAM2BAM over IPA. But these requests are not freed
during disconnect which leads to memory leak and could result in memory
allocation failure in function drivers during next connect. Hence free USB
requests allocated during disconnect to fix memleakissue and also move
allocation of RX/TX usb requests to gbam_connect to avoid crashes due
to NULL pointer dereference.

CRs-Fixed: 840264
Change-Id: Ia4f1d28277a22741defb03086320ffa3895ddfae
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>