staging: mt7621-mmc: Check for nonzero number of scatterlist entries

The buffer descriptor setup loop is correct only if it is setting up at
least one bd struct.  Besides, there is an error if dma_map_sg() returns
0, which is possible and must be handled.

Additionally, remove the BUG_ON() checking sglen, which is unnecessary
because we configure DMA with that constraint during init.

Signed-off-by: George Hilliard <thirtythreeforty@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>