net: atlantic: use intermediate variable to improve readability a bit

This patch syncs up hw_atl_a0.c with an out-of-tree driver, where an
intermediate variable was introduced in a couple of functions to
improve the code readability a bit.

Signed-off-by: Nikita Danilov <ndanilov@marvell.com>
Signed-off-by: Mark Starovoytov <mstarovoitov@marvell.com>
Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>