b43: fix support for 14e4:4321 PCI dev with BCM4321 chipset

It seems Broadcom released two devices with conflicting device id. There
are for sure 14e4:4321 PCI devices with BCM4321 (N-PHY) chipset, they
can be found in routers, e.g. Netgear WNR834Bv2. However, according to
Broadcom public sources 0x4321 is also used for 5 GHz BCM4306 (G-PHY).
It's unsure if they meant PCI device id, or "virtual" id (from SPROM).
To distinguish these devices lets check PHY type (G vs. N).

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Cc: <stable@vger.kernel.org> # 3.16+
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>