mwifiex: cleanup in snmp_mib command preparation code

1) Remove unnecessary switch case usage.
2) Replace "X=cpu_to_le16(le16_to_cpu(X) + Y)" by "le16_add_cpu(X, Y)"
3) Declare "ul_temp" variable as u16 instead of u32 to avoid
unnecessary typecasting

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>