MIPS: Consistently use MIPS_FPU_TYPE for `gdbarch' member

MIPS: Consistently use MIPS_FPU_TYPE for `gdbarch' member

Complement commit 74ed0bb4148e ("Replace current_gdbarch in *mips*"),
<https://sourceware.org/ml/gdb-patches/2008-06/msg00490.html>, and
consistently use the MIPS_FPU_TYPE macro to access the `->mips_fpu_type'
target-dependent `gdbarch' member.  No functional change.

	gdb/
	* mips-tdep.c (mips_gdbarch_init): Use MIPS_FPU_TYPE to access
	`->mips_fpu_type'.