qc_rndis: Add max_pkt_per_xfer and pkt_alignment_factor support

For RNDIS UL aggregation, max_pkt_per_xfer and pkt_alignment_factor
is key information which host expects from device to send aggregated
RNDIS packets. Currently max_pkt_per_xfer and pkt_alignment_factor
are hardcoded with value as 15 and 2 respectively. This change adds
supports which allows setting of these parameters. This changes creates
new sysfs entry for pkt_alignment_factor and uses already available
syfs entry for max_pkt_per_xfer.
e.g.
1.
To enable aggregation of 2 RNDIS Packets with alignment factor 3.
echo 2 >
/sys/devices/virtual/android_usb/android0/f_rndis_qc/max_pkt_per_xfer
echo 3 >
/sys/devices/virtual/android_usb/android0/f_rndis_qc/pkt_alignment_factor
2. If both parameters are not set, it uses default value as 15 and 2 as
default aggregation configuration.
3. For disabling RNDIS UL aggregation,
echo 1 >
/sys/devices/virtual/android_usb/android0/f_rndis_qc/max_pkt_per_xfer

Change-Id: I18d0b720c48c6ed5bf8c08a108736cb4506e2257
Signed-off-by: Mayank Rana <mrana@codeaurora.org>