USB: u_bam: Acquire the correct gbam port ul or dl spinlock

Allocating and freeing tx and rx requests needs to
protect the idle lists using a spinlock. The spinlock
is separate for ul and dl cases. Before allocating
the requests, acquire the respective spinlock.

CRs-Fixed: 734346
Change-Id: Iecb5ed348140b37be732fbfba137eef31d088639
Signed-off-by: Sujeet Kumar <ksujeet@codeaurora.org>