dhcpcd: replace some hard-coded values

Replace some hard-coded values in dhcp.c with references
to macros defined in dhcp.h.

BUG=chromium:330920
TEST=Built the image and deployed to Pixel for sanity tests.

Change-Id: Ifa5bc0734b37bca2676ac1a3b05ef61512e38c5f
Reviewed-on: https://chromium-review.googlesource.com/185086
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
1 file changed