usb: xhci: simplify TDs start and end naming scheme in struct 'xhci_td'

usb: xhci: simplify TDs start and end naming scheme in struct 'xhci_td'

Old names:
 * start_seg - last_trb_seg
 * first_trb - last_trb

New names:
 * start_seg - end_seg
 * start_trb - end_trb

A Transfer Descriptor (TD) in the xhci driver is a data structure that
represents a single transaction to be performed by the USB host controller.
This transaction is defined by TRBs from 'start_trb' in 'start_seg' to
'end_trb' in 'end_seg'.

The terms "start" and "end" were chosen  over "first" and "last" for ease
of searching within the codebase. The ring structure uses 'first_seg' and
'last_seg', while the TD structure uses 'start_seg' and 'end_seg'.

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/20241106101459.775897-24-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>