prism54: fix regression with missing carrier in AP-mode

This fixes a regression introduced by commit 7b463ced

 (prism54: set
carrier flags correctly) which causes the device to come up without
a carrier in AP-mode.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>