NTB: MFV c529aa30: Xeon Doorbell errata workaround

NTB: MFV c529aa30: Xeon Doorbell errata workaround

Modifications to the 14th bit of the B2BDOORBELL register will not be
mirrored to the remote system due to a hardware issue.  To get around
the issue, shrink the number of available doorbell bits by 1.  The max
number of doorbells was being used as a way to referencing the Link
Doorbell bit.  Since this would no longer work, the driver must now
explicitly reference that bit.

This does not affect the xeon_errata_workaround case, as it is not using
the b2bdoorbell register.

Authored by:	Jon Mason
Obtained from:	Linux (Dual BSD/GPL driver)
Sponsored by:	EMC / Isilon Storage Division