drivers: usb: Mark function as static in usbsevseg.c

Mark function my_memlen() as static in misc/usbsevseg.c because it is
not used outside this file.

This eliminates the following warning in misc/usbsevseg.c:
drivers/usb/misc/usbsevseg.c:60:15: warning: no previous prototype for ‘my_memlen’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>