staging/rtl8192e: avoid comparing unsigned type >= 0

There is one remaining warning about a type limit check in rtl8192e:

staging/rtl8192e/rtl819x_TSProc.c:326:14: error: comparison is always true due to limited range of data type [-Werror=type-limits]

This changes a macro into a local function to clarify the types and simplify
the check while removing the warning.

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>