[intelxl] Set maximum frame size to 9728 bytes as per datasheet

[intelxl] Set maximum frame size to 9728 bytes as per datasheet

The PRTGL_SAH register contains the current maximum frame size, and is
not guaranteed on reset to contain the actual maximum frame size
supported by the hardware, which the datasheet specifies as 9728 bytes
(including the 4-byte CRC).

Set the maximum packet size to a hardcoded 9728 bytes instead of
reading from the PRTGL_SAH register.

Signed-off-by: Michael Brown <mcb30@ipxe.org>