mmc: queue: remove BUG_ON for bounce_sg

bounce_sg for mqrq_cur and mqrq_pre are proper
allocated when initializing the queue and will not
be freed before explicitly cleaning the queue. So from
the code itself it should be quite confident to remove
this check. If that BUG_ON take effects, it is mostly
likely the memory is randomly oopsing.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>