Bluetooth: btmrvl: Delete an unnecessary variable initialisation in btmrvl_sdio_register_dev()

The local variable "ret" will be set to an appropriate value a bit later.
Thus omit the explicit initialisation at the beginning.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>