i2c: xiic: Fix RX IRQ busy check

In case the XIIC does TX/RX transfer, make sure no other kernel thread
can start another TX transfer at the same time. This could happen since
the driver only checks tx_msg for being non-NULL and returns -EBUSY in
that case, however it is necessary to check also rx_msg for the same.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>