mmc: sdio_uart: remove meaningless BUG_ON

The code seems quite simple to maintain the sdio_uart_table,
and the insert/remove port from the table are symmetric. If
the BUG_ON occurs, which means serial_core modify the index
or mess up the port sequence anyway.

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