MIPS: Fix build error seen in some configurations

MIPS: Fix build error seen in some configurations

The following build error is seen if CONFIG_32BIT is undefined,
CONFIG_64BIT is defined, and CONFIG_MIPS32_O32 is undefined.

asm/syscall.h: In function 'mips_get_syscall_arg':
arch/mips/include/asm/syscall.h:32:16: error: unused variable 'usp' [-Werror=unused-variable]
cc1: all warnings being treated as errors

Fixes: c0ff3c53d4f9 ('MIPS: Enable HAVE_ARCH_TRACEHOOK')
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: David Daney <david.daney@cavium.com>
Acked-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6160/