drivers: usb: Mark function as static in metro-usb.c

Mark function metrousb_is_unidirectional_mode() in serial/metro-usb.c
because it is not used outside this file.

This eliminates the following warning in serial/metro-usb.c:
drivers/usb/serial/metro-usb.c:57:12: warning: no previous prototype for ‘metrousb_is_unidirectional_mode’ [-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>