iwlwifi: pcie: fix access to scratch buffer

This fixes a pretty ancient bug that hasn't manifested itself
until now.
The scratchbuf for command queue is allocated only for 32 slots
but is accessed with the queue write pointer - which can be
up to 256.
Since the scratch buf size was 16 and there are up to 256 TFDs
we never passed a page boundary when accessing the scratch buffer,
but when attempting to increase the size of the scratch buffer a
panic was quick to follow when trying to access the address resulted
in a page boundary.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Fixes: 38c0f334 ("iwlwifi: use coherent DMA memory for command header")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>