Blackfin: TWI/I2C: fix pure writes

If doing a pure write with register address and data (not a read/write
combo transfer), we don't set the initial transfer length properly which
ends up causing only the register address to be transferred.

While we're here, fix the i2c_write() parameter description of the buffer.

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>