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 d342b95d
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>