cmd_pxe.c: fix strict-aliasing warnings

Without this patch, some versions of gcc (at least ELDK 4.2) complain
about dereferencing type-punned pointers.

Reported-by: Marek Vasut <marex@denx.de>
Signed-off-by: Jason Hobbs <jason.hobbs@calxeda.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: Marek Vasut <marex@denx.de>
Acked-by: Heiko Schocher <hs@denx.de>