io: Fix return type of _inb and _inl

The return type of functions _inb, _inw and _inl are all u16 which looks
wrong.  This patch makes them u8, u16 and u32 respectively.

The original commit text for these does not indicate that these should
be all forced to u16.

Fixes: f009c89d

 ("io: Provide _inX() and _outX()")
Signed-off-by: Stafford Horne <shorne@gmail.com>
Reviewed-by: John Garry <john.garry@huawei.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>