mwl8k: don't touch 'command done' interrupt during firmware load

Since firmware load commands don't generate 'command done' interrupts
like normal commands do, polling for command done interrupts just
unnecessarily slows down the firmware load process.  Removing this bit
of code speeds up loading a typical firmware image from 840 msec to
180 msec.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>