mrf24j40: rework rx handling to async rx handling

This patch prepares that we can do the receive handling inside interrupt
context by using spi_async. This is necessary for introduce a
non-threaded irq handling.

Also we drop the bit setting for "RXDECINV" at register "BBREG1", we do
a driectly full write of register "BBREG1", because it contains the bit
RXDECINV only.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>