Implement support for multiple packets per receive DMA buffer in iwm(4).

Implement support for multiple packets per receive DMA buffer in iwm(4).

We can now clear SINGLE_FRAME_MSK in the receive configuration register.
Linux has not set this bit in years, and Dragonfly stopped setting it
in commit b5eb43f0280bbcfd26af51cf5a4b8e8ff3590b67.
This is a prerequisite for monitor mode since the firmware seems to ignore
the SINGLE_FRAME_MSK bit while running in monitor mode and sends us garbage.

Tested on 7260, 7265, and 8260 devices.