net: ieee802154: adf7242: Avoid redundant RC_READY polling

READ/WRITE register and packet buffer can be done anytime as long
as SPI_READY. Only CMD transactions require RC_READY && SPI_READY.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Stefan Schmidt <stefan@osg.samsung.com>