mraa.c: fix uart mraa_pin_mode_test

switch statement was missing a clause for MRAA_PIN_UART meaning it always
returned False

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
mraa.c: add syslog msg in case we get an unknown pinmode

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
mraa.c: mraa_pin_mode_test should check against b->gpio_count -1

It's already the case for rpi & MMAX but it should be noted that on 1 indexed
boards gpio_count should be +1

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>