mtd: denali: fix incorrect bitmask error in denali_setup_dma

commit 3157d1ed ("mtd: denali: remove unnecessary casts") introduced
an error by using a wrong bitmask.

A uint16_t cast was replaced with & 0xff, should be & 0xffff.

Signed-off-by: Graham Moore <grmoore@opensource.altera.com>
Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>