msm: usb_bam: Fix NULL pointer dereferences

usb_bam_connect() might still be called even if the usb_bam
platform device does not exist. Make sure to check that usb_bam_pdev
is not NULL before attempting to initialize the SPS pipes. Also
clean up other pdev instances in the dev_attr functions.

Change-Id: I0702f6b18eeb67783e9e84d502d64b1042c36c96
Signed-off-by: Jack Pham <jackp@codeaurora.org>