Make smsc75xx recalculate the hard_mtu after adjusting the
hard_header_len.
Without this, usbnet adjusts the MTU down to 1492 bytes, and the host is
unable to receive standard 1500-byte frames from the device.
Inspired by same fix on cdc_eem 78fb72f7936c01d5b426c03a691eca082b03f2b9.
Tested on ARM/Omap3 with EVB-LAN7500-LC.
Signed-off-by: Stephane Fillod <fillods@users.sf.net>
Signed-off-by: David S. Miller <davem@davemloft.net>