IB/ehca: Restructure ehca_set_pagebuf()

Split ehca_set_pagebuf() into three functions depending on MR type
(phys/user/fast) and remove superfluous ehca_set_pagebuf_1().
Signed-off-by: Joachim Fenkes <fenkes@de.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>