b43: avoid uninitialized variable warnings in phy_n

  CC [M]  drivers/net/wireless/b43/phy_n.o
drivers/net/wireless/b43/phy_n.c: In function ‘b43_nphy_set_channel’:
drivers/net/wireless/b43/phy_n.c:3848:47: warning: ‘tabent_r2’ may be used uninitialized in this function
drivers/net/wireless/b43/phy_n.c:3849:47: warning: ‘tabent_r3’ may be used uninitialized in this function
drivers/net/wireless/b43/phy_n.c: In function ‘b43_nphy_poll_rssi.clone.14’:
drivers/net/wireless/b43/phy_n.c:2270:6: warning: ‘save_regs_phy$7’ may be used uninitialized in this function
drivers/net/wireless/b43/phy_n.c:2270:6: warning: ‘save_regs_phy$8’ may be used uninitialized in this function

FWIW, the usage of these variables is goverened by checks that match
their initializations.  So, I think these are actually false warnings.
Still, I would rather avoid the warning SPAM...

Signed-off-by: John W. Linville <linville@tuxdriver.com>