target/mips: Fix data type for offset

Offset can be larger than 16 bit from nanoMIPS,
and immediate field can be larger than 16 bits as well.

Signed-off-by: Yongbok Kim <yongbok.kim@mips.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Aleksandar Markovic <aleksandar.markovic@mips.com>
Signed-off-by: Aleksandar Markovic <aleksandar.markovic@mips.com>