i2c: omap: query STP always when NACK is received

According to I2C specification the NACK should be handled as folowing:
"When SDA remains HIGH during this ninth clock pulse, this is defined as the Not
Acknowledge signal. The master can then gene rate either a STOP condition to
abort the transfer, or a repeated START condition to start a new transfer."
[http://www.nxp.com/documents/user_manual/UM10204.pdf]

The same is recomened by TI I2C wiki:
 http://processors.wiki.ti.com/index.php/I2C_Tips

Currently, the OMAP I2C driver interrupts I2C trunsfer in case of NACK, but
It queries Stop condition OMAP_I2C_CON_REG.STP=1 only if NACK has been received
during the last message transmitting/recieving.
This may lead to stuck Bus in "Bus Busy" until I2C IP reset (idle/enable).

Hence, fix it by querying Stop condition (STP) always when NACK is received.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Hein Tibosch <hein_tibosch@yahoo.es>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>