iwlwifi: mvm: don't send an A-MSDU that is larger than the TXF

The A-MSDU must be smaller than the Transmit FIFO in the
device. Since the size of the TXF can change depending
on the device / firmware compilation mode, take the size
of the FIFO dynamically from the what the firmware tells us.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>