Correct bitoff in big endian OS.
Current code works correctly for 1 byte but not for 2 bytes.

Fixes: 3226aad8 ("sh: support 1 and 2 byte xchg")
Signed-off-by: NPan Xinhui <xinhui.pan@linux.vnet.ibm.com>
Acked-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NRich Felker <dalias@libc.org>