b43: phy.c fix typo in register write

Commit 61bca6eb b43: rewrite A PHY initialization
has a typo, the result of the register read should be masked, not the
register offset.

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