b43: Fix phy_g.c compiler warning

Fix compile warning for non-debug builds:

drivers/net/wireless/b43/phy_g.c: In function ‘b43_gphy_op_recalc_txpower’:
drivers/net/wireless/b43/phy_g.c:3195: warning: unused variable ‘dbm’

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>