msm: usb_bam: Add Null check in connect_pipe()

Pointer returned from call to function dma_alloc_coherent() and
ioremap() may be NULL.
Hence add NULL check to prevent NULL pointer dereference.

Change-Id: I4b3239388ac5d7dd4affbd7db0c38f2a790fce2a
Signed-off-by: Saket Saurabh <ssaurabh@codeaurora.org>