Rename the 'M_B' macro in t4_regs.h to 'CXGBE_M_B'.

This fixes a conflict with the M_B macro in powerpc's
<machine/db_machdep.h> exposed by the recent addition of DDB commands
to the cxgbe driver.

Discussed with: np
Reported by: bz
Sponsored by: Chelsio Communications