ieee802154/atusb: Add function for partial register writes

With this function we can set individual bits in the registers if needed.
With the old SR_VALUE macro we could only set one bit in the register
which was ok for some scenarios but not for all. With this subreg write
function we can now set more bits defined by the mask while not touching
the rest.

We start using it for the current SR_VALUE use case and will use it more
in upcoming patches.

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