usb: usb_bam: Skip setting inactivity timer when it is unchanged

This fix prevents a crash when setting the inactivity timer on
consequent wlan on/off sequence, as a result of unclocked BAM access.

In case inactivity timer value is being set from userspace to
the same value it is currently set to, it is unnecessary to set
the timer again, so skip it.

CRs-fixed: 479379
Change-Id: I1db15e7d801f7cd551de0377284a0dbb9453c95b
Signed-off-by: Amit Blay <ablay@codeaurora.org>