usb: host: xhci: sparse fixes

drivers/usb/host/xhci.c:1826:14: warning: symbol 'xhci_get_block_size' was not declared. Should it be static?
drivers/usb/host/xhci.c:1844:14: warning: symbol 'xhci_get_largest_overhead' was not declared. Should it be static?
drivers/usb/host/xhci-ring.c:2304:36: warning: context imbalance in 'handle_tx_event' - unexpected unlock
drivers/usb/host/xhci-hub.c:425:6: warning: symbol 'xhci_set_remote_wake_mask' was not declared. Should it be static?

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>