i2c: xlp9xx: Handle transactions with I2C_M_RECV_LEN properly

In case of transaction with I2C_M_RECV_LEN set, make sure the driver reads
the first byte and then updates the RX fifo with the expected length. Set
threshold to 1 byte so that driver gets an interrupt on receiving the first byte.
After which the transfer length is updated depending on the received length.
Also report SMBus block read functionality.

Signed-off-by: George Cherian <george.cherian@cavium.com>
Tested-by: dann frazier <dann.frazier@canonical.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>