powerpc/fsl: Fix mfpmr() asm constraint error

mfpmr() needs to be marked always inline, otherwise the compiler will
complain that the rn argument can't be passed to the asm block as an
integer:

  arch/powerpc/include/asm/reg_fsl_emb.h:18:9: warning: 'asm' operand 1 probably does not match constraints
      18 |         asm (".machine push; "
  	     |         ^~~
  arch/powerpc/include/asm/reg_fsl_emb.h:18:9: error: impossible constraint in 'asm'

Mark mtpmr() always inline also to avoid any future problems with it.

Fixes: f01dbd73

 ("powerpc/fsl: Modernise mt/mfpmr")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202403051835.iqLGz996-lkp@intel.com/
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>