dma40: use helpers for channel type check

The somewhat confusing check

	d40c->log_num == D40_PHY_CHAN

and its variants are used in several places to check if a channel is logical or
physical.  Use appropriately named helpers to do this to make the code more
readable.

Acked-by: Per Forlin <per.forlin@stericsson.com>
Acked-by: Jonas Aaberg <jonas.aberg@stericsson.com>
Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>