layers: Fix MSVS value truncation warning

Changed the type of the string validation constants to from char to
uint8_t to fix warning C4309: 'initializing' : truncation of constant
value, due to an implicit unsgined char to signed char conversion.

Change-Id: I86cca4b1a8c3f818a2e7a9a7e38f05ee8cd01150
1 file changed