net: usb: smsc95xx: fix mtu

Make smsc95xx recalculate the hard_mtu after adjusting the
hard_header_len.

Without this, usbnet adjusts the MTU down to 1488 bytes, and the host is
unable to receive standard 1500-byte frames from the device.

Inspired by same fix on cdc_eem 78fb72f7.

Tested on ARM/Beagle.

Signed-off-by: Stephane Fillod <fillods@users.sf.net>
Signed-off-by: David S. Miller <davem@davemloft.net>