drivers/net/e1000.c: Fix GCC 4.6 build warnings

drivers/net/e1000.c: Fix GCC 4.6 build warnings

Fix:
e1000.c: In function 'e1000_read_mac_addr':
e1000.c:1149:2: warning: dereferencing type-punned pointer
will break strict-aliasing rules [-Wstrict-aliasing]

e1000.c:1149:2: warning: dereferencing type-punned pointer
will break strict-aliasing rules [-Wstrict-aliasing]

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Kyle Moffett <Kyle.D.Moffett@boeing.com>
Acked-by: Kyle Moffett <Kyle.D.Moffett@boeing.com>