xhci: Clean up xhci_{alloc,free}_erst() declarations

xhci: Clean up xhci_{alloc,free}_erst() declarations

xhci_alloc_erst() has global scope even though it's only used in
xhci-mem.c.  Declare it static.

xhci_free_erst() was removed by commit b17a57f89f69 ("xhci: Refactor
interrupter code for initial multi interrupter support."), but a
declaration in xhci.h still remains.  Drop it.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20231019102924.2797346-12-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>