USB: u_bam: Separate DL and UL data paths to improve rmnet performance

Current USB RMNET driver uses same spinlock for both DL and UL
data traffic which will hit the performance. Use seperate
spinlocks for DL and UL data traffic for improving throughput
numbers with RMNET interface.

With standalone dataloopback test, increments in throughput numbers
are observed as follows(Mbps):
TCP uplink  	29.7	 55.7
TCP downlink	29.8	 56.8
UDP uplink      16.7     27.8
UDP downlink	20.8	 25.7

Change-Id: I0372aa02940a4331609879a23608cdb15a83bd22
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>