b43: remove set but not used variables 'tx_pwr_state,tmp2'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/wireless/broadcom/b43/phy_n.c: In function 'b43_nphy_op_recalc_txpower':
drivers/net/wireless/broadcom/b43/phy_n.c:5898:7: warning:
 variable 'tx_pwr_state' set but not used [-Wunused-but-set-variable]

drivers/net/wireless/broadcom/b43/phy_n.c:6047:20: warning:
 variable 'tmp2' set but not used [-Wunused-but-set-variable]

'tx_pwr_state' never used since introduction in commit
8615eb28 ("b43: N-PHY: support setting custom TX power")

'tmp2' not used any more since commit c002831a ("b43: N-PHY: use helper
for checking IPA")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>