usb: xhci: fix spelling mistake in Kconfig "firwmare" -> "firmware"

There are two spelling mistakes in the Kconfig text. Fix these.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20200618093224.10179-1-colin.king@canonical.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>