phy: marvell: a3700: Use reg_set_indirect istead of 2 reg_sets

Create a special function for indirect register setting,
reg_set_indirect, and use it instead of the two calls to reg_set.

Signed-off-by: Marek Behun <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>