usb: usb_bam: Code quality checking fix

Fixed a code-quality checking issue:
In the usb_bam_store_inactivity_timer() function, the timer variable
is initalized by the 'strsep()' function return value and there is not
no check whether NULL has been returned. Added an if statement for
verifying non-NULL value before dereferencing this variable.

Change-Id: I9d976cb03aae81e7c25923ee84868279896d8b03
Signed-off-by: Daniel Segal <dsegal@codeaurora.org>