NFC: nfcsim: Simulate lost frames through debugfs entry

This patch allows to simulate the lost of frames exchanged between the 2
nfcsim devices through a control entry in the debugfs and is used as
follow:

 echo n > /sys/kernel/debug/nfcsim/nfcX/dropframe

Where n specifies the number of frames to be dropped between 0 and 255
and nfcX is either nfc0 or nfc1, one of the two nfcsim devices.

In the following example, the next frame that should be sent by the nfc0
device will be dropped and thus not received by the nfc1 device:

 echo 1 > /sys/kernel/debug/nfcsim/nfc0/dropframe

The value of 0 can be used to reset the dropframe counter.

Signed-off-by: Thierry Escande <thierry.escande@collabora.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>