p54: Move rx_mtu to struct bootrec_desc

The patch entitled "[PATCH] p54: Fix sparse warnings" added the __le16
variable rx_mtu to struct bootrec, but it could equally well be placed
in the struct bootrec_desc, which overlays the 'data' section of bootrec.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>