bnx2x: use mask in test_registers() to avoid parity error

Properly mask the value to be written to the register (according to the register size) during the self-test.
Otherwise immediate parity error would be generated.

Signed-off-by: Vladislav Zolotarov <vladz@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>