qla_target: improve qlt_unmap_sg()

Remove the inline attribute.  Modern compilers ignore it and the
function has grown beyond where inline made sense anyway.
Remove the BUG_ON(!cmd->sg_mapped), and instead return if sg_mapped is
not set.  Every caller is doing this check, so we might as well have it
in one place instead of four.

Signed-off-by: Joern Engel <joern@logfs.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>