drivers: i2c: i2c_dw: Fixed integer overflow in i2c_dw_data_ask().

The controller can implement a reception FIFO as deep as 256 bytes.
However, the computation made by the driver code to determine how many
bytes can be asked is stored in a signed 8-bit variable called rx_empty.

If the reception FIFO depth is greater or equal to 128 bytes and the FIFO
is currently empty, the rx_empty value will be 128 (or more), which
stands for a negative value as the variable is signed.

Thus, the later code checking if the FIFO is full will run while it should
not and exit from the i2c_dw_data_ask() function too early.

This hangs the controller in an infinite loop of interrupt storm because
the interrupt flags are never cleared.

Storing the rx_empty empty on a signed 32-bit variable instead of a 8-bit
one solves the issue and is compliant with the controller hardware
specifications of a maximum FIFO depth of 256 bytes.

It has been agreed with upstream maintainers to change the type of the
variables tx_empty, rx_empty, cnt, rx_buffer_depth and tx_buffer_depth to
plain int because it is most effectively handled by the CPUs. Using 8-bit
or 16-bit variables had no meaning here.

Signed-off-by: Adrien Ricciardi <aricciardi@baylibre.com>