b43: LP-PHY: Fix setting TX power control mode during RC calibration

Call set_tx_power_control with a LPPHY_TXPCTL rather than an
LPPHY_TX_PWR_CTL_CMD_MODE.

Signed-off-by: Gábor Stefanik <netrolller.3d@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>