usb: xhci: refactor trb_in_td() to be static

usb: xhci: refactor trb_in_td() to be static

Relocate trb_in_td() and marks it as static, as it's exclusively utilized
in xhci-ring.c. This adjustment lays the groundwork for future rework of
the function.

The function's logic remains unchanged; only its access specifier is
altered to static and a redundant "else" is removed on line 325
(due to checkpatch.pl complaining).

Signed-off-by: Niklas Neronin <niklas.neronin@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20250306144954.3507700-11-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>