usb: gadget: modify gserial and grmnet structures

Rename send_cpkt_request to send_encap_cmd & send_cbits_tomodem
to notify_modem.
Update notify_modem and send_encap_cmd functions' parameter to
void pointer.
Remove struct grmnet pointer parameter from send_encap_cmd
function.

Above changes are done to come up with generic callbacks registered
by transport drivers for both the structures.

Change-Id: I149386186d42a5c77adca2da2a7b4bdcf4c01e88
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>