Compiling the rtlwifi drivers for ARM with gcc -Wextra warns about lots of
incorrect code that results from 'char' being unsigned here, e.g.
staging/rtl8192u/r8192U_core.c:4150:16: error: comparison is always false due to limited range of data type [-Werror=type-limits]
staging/rtl8192u/r8192U_dm.c:646:50: error: comparison is always false due to limited range of data type [-Werror=type-limits]
This patch changes all uses of 'char' in this driver that refer to
8-bit integers to use 's8' instead, which is signed on all architectures.