MIPS: BPF: Replace __mips_isa_rev with MIPS_ISA_REV

MIPS: BPF: Replace __mips_isa_rev with MIPS_ISA_REV

Remove the need to check that __mips_isa_rev is defined by using the
newly added MIPS_ISA_REV.

Signed-off-by: Matt Redfearn <matt.redfearn@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Paul Burton <paul.burton@mips.com>
Cc: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/18677/
Signed-off-by: James Hogan <jhogan@kernel.org>