mwifiex: avoid exporting mwifiex_send_cmd

This is a follow-up patch for commit 21c5c83c
("mwifiex: support sysfs initiated device coredump").

Let us avoid exporting mwifiex_send_cmd and instead use a utility
function mwifiex_fw_dump_event to achive the work.

Signed-off-by: Ganapathi Bhat <gbhat@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>