drivers/scsi/cxgbi: fix build with EXTRA_CFLAGS

EXTRA_CFLAGS are intended to be used on the command line, not by Kbuild.
In case of cxgbi drivers, use of EXTRA_CFLAGS results in a compilation
failure:

  drivers/scsi/cxgbi/cxgb4i/cxgb4i.c:24:21: fatal error: t4_regs.h: No such file or directory

when building like:

$ make drivers/scsi/cxgbi/ EXTRA_CFLAGS=-Wwhatever

Use ccflags-y instead of EXTRA_CFLAGS.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>