samples: mesh_demo: Fix  Fix warning when building with newlib
samples: mesh_demo: Fix  Fix warning when building with newlib

If we use newlib the isdigit (and other similar functions) return an
error as char can possibly be viewed as signed:

usr/include/ctype.h:57:54: error: array subscript has type ‘char’ [-Werror=char-subscripts]
 #define __ctype_lookup(__c) ((__ctype_ptr__+sizeof(""[__c]))[(int)(__c)])

Explicity cast to unsigned char so we deal with both this warning and
possible warning when -Wpointer-sign is enabled.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

ldts/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,nashif/zephyr,explora26/zephyr,punitvara/zephyr,Vudentz/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,ldts/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,ldts/zephyr,finikorg/zephyr,kraj/zephyr,Vudentz/zephyr,finikorg/zephyr,kraj/zephyr,galak/zephyr,punitvara/zephyr,nashif/zephyr,galak/zephyr,ldts/zephyr,finikorg/zephyr,kraj/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,explora26/zephyr,kraj/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,explora26/zephyr