xtensa: use for_each_sg()

xtensa: use for_each_sg()

This replaces the plain loop over the sglist array with for_each_sg()
macro which consists of sg_next() function calls.  Since xtensa doesn't
select ARCH_HAS_SG_CHAIN, it is not necessary to use for_each_sg() in
order to loop over each sg element.  But this can help find problems
with drivers that do not properly initialize their sg tables when
CONFIG_DEBUG_SG is enabled.

Signed-off-by: Akinobu Mita <[email protected]>
Cc: Chris Zankel <[email protected]>
Cc: Max Filippov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>