xhci: Clean up stale comment on ERST_SIZE macro

xhci: Clean up stale comment on ERST_SIZE macro

Commit ebd88cf50729 ("xhci: Remove unused defines for ERST_SIZE and
ERST_ENTRIES") removed the ERST_SIZE macro but retained a code comment
explaining the quantity chosen in the macro.

Remove the code comment as well.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20231019102924.2797346-11-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>