b43: fix NULL pointer dereference in b43_phy_copy()

phy_read and phy_write are not set for every phy any more sine this:
commit d342b95
Author: Rafał Miłecki <zajec5@gmail.com>
Date:   Thu Jul 31 21:59:43 2014 +0200

    b43: don't duplicate common PHY read/write ops

b43_phy_copy() accesses phy_read and phy_write directly and will fail
with some phys. This patch fixes the regression by using the
b43_phy_read() and b43_phy_write() functions which should be used for
read and write access.

This should fix this bug report:
https://bugzilla.kernel.org/show_bug.cgi?id=87731

Reported-by: Volker Kempter <v.kempter@pe.tu-clausthal.de>
Tested-by: Volker Kempter <v.kempter@pe.tu-clausthal.de>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/b43/phy_common.c

@@ -300,9 +300,7 @@ void b43_phy_write(struct b43_wldev *dev, u16 reg, u16 value)
 
 void b43_phy_copy(struct b43_wldev *dev, u16 destreg, u16 srcreg)
 {
-	assert_mac_suspended(dev);
-	dev->phy.ops->phy_write(dev, destreg,
-		dev->phy.ops->phy_read(dev, srcreg));
+	b43_phy_write(dev, destreg, b43_phy_read(dev, srcreg));
 }
 
 void b43_phy_mask(struct b43_wldev *dev, u16 offset, u16 mask)