mraa.c: fix unsigned int comparison by usign UINT_MAX instead of -1
mraa.c: fix unsigned int comparison by usign UINT_MAX instead of -1

This works since both are essentially 'impossible' values.

Signed-off-by: Brendan Le Foll <3e0e8c4069934e8b47d99704927a85c66c0ceb33@intel.com>

yongli3/mraa,Propanu/mraa,petreeftime/mraa,Jon-ICS/mraa,andreivasiliu2211/mraa,stefan-andritoiu/mraa-gpio-chardev,petreeftime/mraa,arunlee77/mraa,yongli3/mraa,jontrulson/mraa,Propanu/mraa,yongli3/mraa,alext-mkrs/mraa,arunlee77/mraa,malikabhi05/mraa,alext-mkrs/mraa,intel-iot-devkit/mraa,spitfire88/mraa,Propanu/mraa,petreeftime/mraa,malikabhi05/mraa,w4ilun/mraa,malikabhi05/mraa,stefan-andritoiu/mraa-gpio-chardev,ncrastanaren/mraa,alext-mkrs/mraa,arfoll/mraa,yongli3/mraa,stefan-andritoiu/mraa-gpio-chardev,ncrastanaren/mraa,g-vidal/mraa,jontrulson/mraa,w4ilun/mraa,malikabhi05/mraa,Jon-ICS/mraa,stefan-andritoiu/mraa,jontrulson/mraa,KurtE/mraa,g-vidal/mraa,arfoll/mraa,arfoll/mraa,Hbrinj/mraa,w4ilun/mraa,jontrulson/mraa,Propanu/mraa,ProfFan/mraa,andreivasiliu2211/mraa,spitfire88/mraa,sergev/mraa,Propanu/mraa,sergev/mraa,g-vidal/mraa,malikabhi05/mraa,whbruce/mraa,intel-iot-devkit/mraa,Hbrinj/mraa,ncrastanaren/mraa,jontrulson/mraa,ProfFan/mraa,yongli3/mraa,KurtE/mraa,whbruce/mraa,ncrastanaren/mraa,stefan-andritoiu/mraa,ProfFan/mraa,arfoll/mraa,g-vidal/mraa,stefan-andritoiu/mraa,alext-mkrs/mraa,intel-iot-devkit/mraa,KurtE/mraa,sergev/mraa,alext-mkrs/mraa,stefan-andritoiu/mraa-gpio-chardev,spitfire88/mraa,andreivasiliu2211/mraa,stefan-andritoiu/mraa,KurtE/mraa,Hbrinj/mraa,stefan-andritoiu/mraa-gpio-chardev,ncrastanaren/mraa,intel-iot-devkit/mraa,w4ilun/mraa,whbruce/mraa,g-vidal/mraa,petreeftime/mraa,intel-iot-devkit/mraa,sergev/mraa,Hbrinj/mraa,arunlee77/mraa,stefan-andritoiu/mraa,arfoll/mraa,spitfire88/mraa,arunlee77/mraa,whbruce/mraa,Jon-ICS/mraa,sergev/mraa,ProfFan/mraa,andreivasiliu2211/mraa,KurtE/mraa