mwifiex: support sysfs initiated device coredump

Since commit 3c47d19f ("drivers: base: add coredump driver ops")
it is possible to initiate a device coredump from user-space. This
patch adds support for it adding the .coredump() driver callback.
As there is no longer a need to initiate it through debugfs remove
that code.

Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>