Staging: rtl8192su, rtl8192u: use min_t() in store_debug_level()

sizeof() returns a size_t but the other types involved
are unsigned long, so using min() results in a warning.

As sizeof() is called on an 11 character buffer defined
immediately above unsigned long is obviously wide enough
for the result.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>