net: mhi: Increase the default MTU from 16K to 32K

Most of the Qualcomm endpoint devices are supporting 32K MTU for the
UL (Uplink) and DL (Downlink) channels. So let's use the same value
in the MHI NET driver also. This gives almost 2x increase in the throughput
for the UL channel.

Below is the comparision:

iperf on the UL channel with 16K MTU:

[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-10.0 sec   353 MBytes   296 Mbits/sec

iperf on the UL channel with 32K MTU:

[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-10.0 sec   695 MBytes   583 Mbits/sec

Cc: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>