MIPS: Make declarations and definitions of tlbmiss_handler_setup_pgd match.

tlbmiss_handler_setup_pgd is run-time generated code and it was convenient
to pretend the symbol was an array in the generator but a function for
the users.  LTO gcc won't tolerate this kind of lie anymore so solve the
problem through a cast and function pointer instead.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>