dmaengine: PL08x: Fix reading the byte count in cctl

There are more fields than just SWIDTH in CH_CONTROL register, so read
register value must be masked in addition to shifting.

Signed-off-by: Alban Bedel <alban.bedel@avionic-design.de>
Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>