microblaze: Fix sparse warning - consistent_alloc function

Warning in dma.c was caused by incorrect type in consistent_alloc function.

Warning log:
CHECK   arch/microblaze/kernel/dma.c
arch/microblaze/kernel/dma.c:53:26: warning: incorrect type in argument 1 (different base types)
arch/microblaze/kernel/dma.c:53:26:    expected int [signed] gfp
arch/microblaze/kernel/dma.c:53:26:    got restricted unsigned int [usertype] flag

Signed-off-by: Michal Simek <monstr@monstr.eu>