GCC4.6: Squash warnings in ahci.c

ahci.c: In function 'ahci_port_start':
ahci.c:401: warning: format '%x' expects type 'unsigned int', but argument 2 has
type 'struct ahci_cmd_hdr *'

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Mike Frysinger <vapier@gentoo.org>