amd-xgbe: Fix mask appliciation for Clause 37 register

The application of a mask to clear an area of a clause 37 register value
was not properly applied. Update the code to do the proper application
of the mask.

Reported-by: Marion &amp; Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>