clean up reading of ICR register in FEC driver

On the MCF5272, there is no need to read the ICR before writing it :
the bit 4n+3 is a write-enable for the bits 4n,4n+1 and 4n+2.

Signed-off-by: Philippe De Muyter <phdm@macqel.be>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>