[LIBERTAS]: fix interrupts in CF driver

The following patch fixes the tx transmit timeout problem, which is
caused by the interrupts being incorrectly check and masked. The patch
moves the interrupt masking code so that interrupts are enabled only
when the driver is registered and only disabled when the driver is
unregistered.

Signed-off-by: Ryan Mallon <ryan@bluewatersys.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>