GRUB: only load xhci from grub.cfg

don't put it in the install modules.

this works around a hanging issue on haswell thinkpads.

when any usb device is inserted, GRUB will sometimes
hang if started from the SeaBIOS payload, *while* the
USB device is plugged in.

plugging in the USB device after GRUB starts worked.
it will have to be investigated more at a later date,
but this simply configuration change works.

the xhci module is already loaded explicitly, in grub.cfg

Signed-off-by: Leah Rowe <leah@libreboot.org>

config/data/grub/module/xhci

@@ -8,6 +8,7 @@ usbserial_ftdi \
 usbserial_pl2303 \
 usbserial_usbdebug \
 video_colors \
+xhci \
 "
 # Modules (and always loaded)
 grub_modules=" \
@@ -35,7 +36,6 @@ crypto \
 cryptodisk \
 diskfilter \
 echo \
-xhci \
 ehci \
 eval \
 exfat \