dmaengine: ste_dma40: Remove home-brew MAX() macro

The current implementation of the DMA40's local MAX() macro evaluates
its arguments more times than is necessary and it not type checked.
This patch strips replaces it with a new, more efficient constant.

Cc: Dan Williams <djbw@fb.com>
Cc: Per Forlin <per.forlin@stericsson.com>
Cc: Rabin Vincent <rabin@rab.in>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Reported-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>