usb: xhci: remove redundant variable 'erst_size'

'erst_size' represents the maximum capacity of entries that ERST can hold,
while 'num_entries' indicates the actual number of entries currently held
in the ERST. These two values are identical because the xhci driver does
not support ERST expansion. Thus, 'erst_size' is removed.

Suggested-by: Mathias Nyman <mathias.nyman@linux.intel.com>
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/20240429140245.3955523-7-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>