Input: adp5588-keys - fix check on return code

Input: adp5588-keys - fix check on return code

During adp5588_setup(), we read all the events to clear the event FIFO.
However, adp5588_read() just calls i2c_smbus_read_byte_data() which
returns the byte read in case everything goes well. Hence, we need to
explicitly check for a negative error code instead of checking for
something different than 0.

Fixes: e960309ce318 ("Input: adp5588-keys - bail out on returned error")
Cc: stable@vger.kernel.org
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240920-fix-adp5588-err-check-v1-1-81f6e957ef24@analog.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>