powerpc/xive: Remove (almost) unused macros

The GETFIELD and SETFIELD macros in xive-regs.h aren't used except for
a single instance of GETFIELD, so replace that and remove them.

These macros are also defined in vas.h, so either those should be
eventually replaced or the macros moved into bitops.h.

Signed-off-by: Russell Currey <ruscur@russell.cc>
[mpe: Rewrite the assignment to 'he' to avoid ffs() etc.]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>