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

The variable "payload" will eventually be set to an appropriate pointer
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>