drivers: net: wireless: b43: Fix possible NULL ptr dereference

On the ternary expression the 'e' variable could be NULL dereferenced,
when b43_nphy_get_rf_ctl_over_rev7 function returns NULL.

Signed-off-by: Felipe Pena <felipensp@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>