dmaengine: PL08x: Refactor pl08x_getbytes_chan() to lower indentation

Further patch will introduce support for PL080S, which requires some
things to be done conditionally, thus increasing indentation level of
some functions even more.

This patch reduces indentation level of pl08x_getbytes_chan() function
by inverting several conditions and returning from function wherever
possible.

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>