hikey960: fix the calculation in boardid
hikey960: fix the calculation in boardid

Since the type of ADC value is always unsigned int, don't
need to check the value with negative value.

Signed-off-by: Haojian Zhuang <979139d45d262f0480136b0da512e0afa0fe79d3@linaro.org>

lsigithub/arm-trusted-firmware_public,achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public