e1000: fix compile warning

Get rid of compiler warning:
e1000.c: In function 'e1000_transmit':
e1000.c:5028: warning: passing argument 1 of 'virt_to_phys' discards qualifiers from pointer target type

Signed-off-by: Wolfgang Denk <wd@denx.de>
Acked-by: Stefan Roese <sr@denx.de>