usb: gadget: u_ether: add tx multipler module param

Usually tx rates are higher than rx rates and it deserve higher
number of requests. For LTE category 6 data rates of 300Mbps (about
30 1.5KB packets per milli-sec), the current maximum of 40 usb
request may not be sufficient. At this rate and with interrupt
moderation of interconnect, data can be very bursty. Add module
parameter to configure number of tx usb requests that can changed
for different driver configurations.

Change-Id: I2a380274b62bbcfa46d1147ff463b461229431cd
Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>