wl12xx: alloc buffer in driver_state_read to prevent compilation warning

wl12xx: alloc buffer in driver_state_read to prevent compilation warning

When compiling wl12xx for x86, there was a warning complaining about
the size of the buffer we were allocating in the stack:

drivers/net/wireless/wl12xx/debugfs.c: In function 'driver_state_read':
drivers/net/wireless/wl12xx/debugfs.c:380:1: warning: the frame size of 1040 bytes is larger than 1024 bytes

To prevent this, allocate the buffer in the heap instead.

Signed-off-by: Luciano Coelho <coelho@ti.com>