pl08x_width function does not handle rest of enums for DMA_SLAVE_BUSWIDTH_xxxx
which causes gcc to emit below warining

drivers/dma/amba-pl08x.c: In function 'pl08x_width':
	drivers/dma/amba-pl08x.c:1119: warning: enumeration value
	'DMA_SLAVE_BUSWIDTH_UNDEFINED' not handled in switch
	drivers/dma/amba-pl08x.c:1119: warning: enumeration value
	'DMA_SLAVE_BUSWIDTH_8_BYTES' not handled in switch

this patch adds a default case which returns error
Signed-off-by: Vinod Koul <vinod.koul@intel.com>