usb: gadget: ether: Reduce max number of scatter/gather packets

The DWC3 controller cannot process more than 7 chained TRBs if the
total payload is less than maxPacketSize. Since this can occur if multiple
small network packets (e.g. TCP ACKs) are being aggregated together using
u_ether's scatter-gather optimization, reduce the maximum number of packet
aggregation accordingly. For RmNet, instead of 10, change it to 7.
For RNDIS, since each packet becomes a header+payload pair, the maximum can
only be 3.

CRs-fixed: 755801
Change-Id: I96b567bbb64bfd9ef0c87ad455162b730eba17b5
Signed-off-by: Azhar Shaikh <azhars@codeaurora.org>