usbmon: Remove erroneous __exit

mon_bin_exit() and mon_text_exit() are called from __init code, so don't mark
them as __exit.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>