i2c: i2c-omap: Fix BUFSTAT_REG reading

The number of bytes to be received is read from wrong
place with all OMAPs with highspeed I2C support,
which involves a FIFO and BUFSTAT_REG. It is the 6
bits starting from the bit 8 in the BUFSTAT_REG
that indicate this amount of bytes to be read.
Moreover, only the 6 LSB:s are relevant for the
TXSTAT field.

Signed-off-by: Eero Nurkkala <ext-eero.nurkkala@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>