dmaengine: dmatest: Add check for supported buffer count (sg_buffers)

When using dmatest with sg_buffers=128 I stumbled upon the problem, that
the "map_cnt" variable of "struct dmaengine_unmap_data" was set to 0.
"map_cnt" is an "u8" variable, resulting in an overrun when its
value is set to src_cnt + dst_cnt, to twice the sg_buffer value.

This patch adds a small check to dmatest, so that this confusing error
is detected and the test is aborted.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Kedareswara rao Appana <appanad@xilinx.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>