USB: gadget: u_bam: Add UL pending data threshold in bytes

Current 'bam_pending_limit' parameter limits the number of UL
packets pending with BAM dmux. For different network modes
(WCMDA/LTE) there could be a limit to the maximum size data
burst supported. As these bursts are defined in terms of
bytes, add a configurable parameter in driver to allow user
to change this as per the modem requirement.
Added module parameter: 'bam_pending_bytes_limit' under
'/sys/module/g_android/parameters'.
UL packets for bam_dmux are stopped as soon as either the
bytes_limit or packets_limit is reached.

Change-Id: Iba7df5461b15753d16245da4d2c60f39f7ba0a04
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>