IB/ipath: Generalize some xxx_SHIFT macros

In preparation for upcoming chips that have different values for
INFINIPATH_R_PORTENABLE_SHIFT, INFINIPATH_R_INTRAVAIL_SHIFT,
INFINIPATH_R_TAILUPD_SHIFT, and portcfg_shift, remove the shared
#defines and use device-specific variables instead.

Signed-off-by: Dave Olson <dave.olson@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>