i2c: designware: Avoid overwriting the cmd_data register

Make sure the driver writes the cmd_data register only once per
read transfer instead of doing so potentially repeatedly.

In case the read transfer didn't finish quickly enough, the loop
in the driver code would spin fast enough to write the same value
into the cmd_data register again before re-checking whether the
transfer completed, which would cause another spurious read transfer
on the bus.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefan Roese <sr@denx.de>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Chin Liang See <clsee@altera.com>