Fix ppc 32 build failure as reported here:

http://kisskb.ellerman.id.au/kisskb/buildresult/11663513/

The error is as follows:

arch/powerpc/include/asm/floppy.h:142:20: error: 'isa_bridge_pcidev' undeclared
(first use in this function)

This is happening since floppy.o is enabled by BLK_DEV_FD which depends on
ARCH_MAY_HAVE_PC_FDC which is in-turn enabled if PPC_PSERIES=n.

The following commit changes the dependency so that ARCH_MAY_HAVE_PC_FDC is
dependent exclusively on PCI since otherwise it will not compile.
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
CC: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>