net: macb: fix the following building warning

fix the following building warning
---8>---
macb.c: In function 'macb_init':
macb.c:400:14: warning: 'phydev' may be used uninitialized in this function
macb.c:377:21: note: 'phydev' was declared here
---<8---

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>