Input: hp_sdc_rtc - fix test in hp_sdc_rtc_read_rt()

If left unsigned the hp_sdc_rtc_read_i8042timer() return value will not
be checked correctly.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>