target/xtensa: fix mapping direction in read/write simcalls

Read and write simcalls map physical memory to access I/O buffers, but
'read' simcall need to map it for writing and 'write' simcall need to
map it for reading, i.e. the opposite of what they do now. Fix that.

Cc: qemu-stable@nongnu.org
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>