As per IEEE Std 802.11ac-2013, 'Maximum A-MPDU Length Exponent' field
value is in the range of 0 to 7. Previous implementation assumed EXP0 to
be the maximum length (bits 23, 24 and 25 set) what is incorrect.
This patch adds options to set it up within the 0 to 7 range.
Signed-off-by: Bartosz Markowski <bartosz.markowski@tieto.com>