m25p80.c: Return state to IDLE after COLLECTING

Default to moving back to the IDLE state after the COLLECTING_DATA
state. For a well behaved guest this patch has no consequence, but
A bad guest could crash QEMU by using one of the erase commands
followed by a longer than 5 byte argument (undefined behaviour).

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
