tcg/mips: fix store softmmu slow path

Commit 9d8bf2d1 moved the softmmu slow path out of line and introduce a
regression at the same time by always calling tcg_out_tlb_load with
is_load=1. This makes impossible to run any significant code under
qemu-system-mips*.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-stable@nongnu.org
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>