mediatek: cast literal value to char

mediatek: cast literal value to char

Or the comparison against a signed char is always true, because the
literal 0xaa is treated as an unsigned int, to which the signed char is
casted during comparison. 0xaa is above the positive values of a signed
char and negative signed char values result in values larger than 0xaa
when casted to unsigned int.

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>