bnxt_en: Fix context memory setup for 64K page size.

There was a typo in the code that checks for 64K BNXT_PAGE_SHIFT in
bnxt_hwrm_set_pg_attr().  Fix it and make the code more understandable
with a new macro BNXT_SET_CTX_PAGE_ATTR().

Fixes: 1b9394e5 ("bnxt_en: Configure context memory on new devices.")
Reviewed-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>