Bluetooth: btmrvl: support sysfs initiated firmware 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 in btmrvl_sdio adding the .coredump()
driver callback. This makes dump through debugfs obsolete so removing
it.

Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>