marvell: comphy_a3700: fix bitmask

Obviously the mask for the rx and tx select field cannot be right,
as it would overlap in one and exceed the 32-bit register in the other
case. From looking at the neighbouring bits it looks like the mask
should be really 4 bits wide instead of 8.

Pointed out by a GCC 6.2 (default) warning.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Stefan Roese <sr@denx.de>