IB/mthca: Use an enum for HCA page size

Use a named enum for the HCA's internal page size, rather than having
magic values of 4096 and shifts by 12 all over the code.  Also, fix
one minor bug in EQ handling: only one HCA page is mapped to the HCA
during initialization, but a full kernel page is unmapped during
cleanup.  This might cause problems when PAGE_SIZE != 4096.

Signed-off-by: Ishai Rabinovitz <ishai@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>