net: usb: smsc75xx: fix mtu

net: usb: smsc75xx: fix mtu

[ Upstream commit a99ff7d0123b19ecad3b589480b6542716ab6b52 ]

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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>