misc/isl29020: signedness bug in als_sensing_range_store()

"ret_val" is supposed to be signed here or the error handling breaks.
Also we should check the return value from i2c_smbus_read_byte_data().

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>