usb: gadget: f_gsi: Increase num of buffers to 50 for ECM and RmNet

Currently the ECM interface has 15 data buffers allocated by the
driver which can be used by IPA. In some telematics platform
concurrency scenarios like Ethernet and USB together, this number
is not enough for IPA to keep packets in its stack and service
the Ethernet pipe. Hence the packets get dropped leading to lower
throughput on both the pipes. Fix this by increasing the number
of buffers to 50 for relevant interfaces.

Change-Id: I65e0282143d489ab9f6b9fcc640cad7808adb68d
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>