staging/rdma/hfi1: rework is_a0() and is_bx()

The current is_bx() will incorrectly match on other steppings.

is_a0() is removed in favor of is_ax().

Reviewed-by: Mark Debbage <mark.debbage@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Jubin John <jubin.john@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>