dma: at_hdmac: fix invalid remaining bytes detection

Found using smatch:
drivers/dma/at_hdmac.c:299 atc_get_bytes_left() warn: unsigned
'atchan->remain_desc' is never less than zero.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>