USB: Mark EHCI LPM functions as __maybe_unused

ehci_lpm_set_da and ehci_lpm_check are EHCI 1.1 specific functions which
are not used on many platforms but do generate annoying gcc warnings

Signed-off-by: Maksim Rayskiy <mrayskiy@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>