More correctly handle the case where plain char is signed

In _FAT_directory_lfnLength, the check 'ch < 0x20' incorrectly catches
any extended UTF-8 bytes if the plain 'char' type is signed:

  0x80 < 0x20 ... 0xEF < 0x20

are interpreted as (negative value < 32). The check for 0xF0 and above
also fails to work properly due to 0xF0 being negative as well.

Author: Nebuleon

source/directory.c

@@ -98,7 +98,8 @@ static int _FAT_directory_lfnLength (const char* name) {
 	}
 	// Make sure the name doesn't contain any control codes or codes not representable in UCS-2
 	for (i = 0; i < nameLength; i++) {
-		if (name[i] < 0x20 || name[i] >= ABOVE_UCS_RANGE) {
+		unsigned char ch = (unsigned char) name[i];
+		if (ch < 0x20 || ch >= ABOVE_UCS_RANGE) {
 			return -1;
 		}
 	}