staging/rtl8192u: use s8 instead of char

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.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>