p54: fix oops caused by bad eeproms

This patch fixes a bug that could occur, if it the eeprom is incomplete or partly corrupted.

BUG: unable to handle kernel NULL pointer dereference at 00000008
IP: p54_assign_address+0x108/0x15d [p54common]
Oops: 0002 [#1] SMP
Pid: 12988, comm: phy1 Tainted: P        W  2.6.28-rc6-wl #3
RIP: 0010: p54_assign_address+0x108/0x15d [p54common]
 [...]
Call Trace:
 p54_alloc_skb+0xa3/0xc0 [p54common]
 p54_scan+0x37/0x204 [p54common]
 [...]

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>