ath9k: drop negativity checks for unsigned values coming from kstrtoul()

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=80471
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=80481


Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Andrey Utkin <andrey.krieger.utkin@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>