net: e1000: Fix build warnings for 32-bit

commit 6497e37a "net: e1000: Support 64-bit physical address" causes
compiler warnings on 32-bit U-Boot build below.

drivers/net/e1000.c: In function 'e1000_configure_tx':
drivers/net/e1000.c:4982:2: warning: right shift count >= width of type [enabled by default]
drivers/net/e1000.c: In function 'e1000_configure_rx':
drivers/net/e1000.c:5126:2: warning: right shift count >= width of type [enabled by default]

This commit fixes the build warnings.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>