b43: Fix LO calibration txctl reg value

This patch expands the parenthesis in the txctl reg write
of the LO calibration to enforce precedence rules.

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