IB/ehca: Fix ehca_encode_hwpage_size() and alloc_fmr()

Simplify ehca_encode_hwpage_size(), fixing an infinite loop for pgsize == 0
in the process. Fix the bug in alloc_fmr() that triggered the loop.

Signed-off-by: Joachim Fenkes <fenkes@de.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>