staging: rtl8192e: fix wrong assignment

This patch addresses a spatch warning on assigning a negative
value to a unsigned integer.Similar patch has been submitted by
Larry Finger earlier to silence the same spatch warning in another
file.

Signed-off-by: Hari Prasath Gujulan Elango <hgujulan@visteon.com>
Acked-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>