mwl8k: interrupt handling changes

We do not need to enable all the interrupts in mwl8k_probe_hw.
We need to enable only MWL8K_A2H_INT_OPC_DONE interrupt for sending
commands to the firmware. Keep the other interrupts masked in
mwl8k_probe_hw. Also, in mwl8k_start, where we expect other interrupts,
enable only those interrupts we are interested in.

Signed-off-by: Nishant Sarmukadam <nishants@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>