usbtest: make module param pattern writeable

Allow module_param to be writeable. This allows us to change
the parameter if usbtest is built-in in the kernel.

Signed-off-by: Vikram Pandita <vikram.pandita@ti.com>
Signed-off-by: Anand Gadiyar <gadiyar@ti.com>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>