cxgbe/iw_cxgbe: Replace a magic constant with something more readable
(and accurate).

T4 and later have an extra bit for page shift so the maximum page size
is 8TB (shift of 12 + 31) instead of 128MB (12 + 15). This saves space
in the chip's PBL (physical buffer list) when registering very large
memory regions.

MFC after: 3 days
Sponsored by: Chelsio Communications