s390/raw3270: make raw3270_buffer_address() accept x/y coordinates

All callers of raw3270_buffer_address() are calculating the offset
from some x/y coordinates. Move that calculation inside of the
function, so user can pass the x/y values directly. Note that
negative values are relative to the end-of-line or end-of-screen.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Tested-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>