dmaengine: ste_dma40: Separate Logical Global Interrupt Mask (GIM) unmasking

During the initial setup of a logical channel, it is necessary to unmask
the GIM in order to receive generated terminal count and error interrupts.
We're separating out this required code so it will be possible to move
the remaining code in d40_phy_cfg(), which is mostly runtime configuration
into the runtime_config() routine.

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>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>