i2c: xlr: fix extra read/write at end of rx transfer

The BYTECNT register holds the transfer size minus one.  Setting it to
the correct value removes the need for a dummy read/write at the end of
each transfer.  As zero-length transfers are not supported, do not
advertise I2C_FUNC_SMBUS_QUICK.

In other words, this patch makes the driver transfer the number of bytes
requested unless this is zero, which is not supported by the hardware
and is thus refused.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>