mwifiex: increase the number of nodes in command pool

Command nodes are increased from 20 to 50. Now we can always
scan 1 channel per scan command to avoid traffic delay/loss in
connected state. We will get rid of *CHANNEL_PER_SCAN_CMD macros
used due to command node constraints.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>