drm: bridge: sii8620: fix possible off-by-one

The next call to sii8620_burst_get_tx_buf will result in off-by-one
When ctx->burst.tx_count + size == ARRAY_SIZE(ctx->burst.tx_buf). The same
thing happens in sii8620_burst_get_rx_buf.

This patch also change tx_count and tx_buf to rx_count and rx_buf in
sii8620_burst_get_rx_buf. It is unreasonable to check tx_buf's size and
use rx_buf.

Fixes: e19e9c69 ("drm/bridge/sii8620: add support for burst eMSC transmissions")
Signed-off-by: Hangyu Hua <hbh25y@gmail.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220518065856.18936-1-hbh25y@gmail.com