bnxt_en: Use register window 6 instead of 5 to read the PHC

Some older Broadcom debug tools use window 5 and may conflict, so switch
to use window 6 instead.

Fixes: 118612d5

 ("bnxt_en: Add PTP clock APIs, ioctls, and ethtool methods")
Reviewed-by: Andy Gospodarek <gospo@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>