MIPS: __delay ABI-dependent subtraction simplification

This small update to the previous fix to __delay removes a conditional
around the ABI-dependent subtraction operation within an inline asm in
favor to the standard <asm/asm.h> LONG_SUBU macro.  No change in code
produced.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6703/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>