[SSB] Initialise dma_mask for SSB_BUSTYPE_SSB devices

For SSB_BUSTYPE_SSB type devices, we need to initialize dma_mask using
coherent_dma_mask so that calls to dma_set_mask() succeed.

It fixes the regression on the b44 driver introduced by commit
f225763a

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>