Input: sermouse - improve protocol error recovery

When using MS protocol the driver should wait for a byte with
bit 6 set before assuming that it sees beginning of a data packet.
This should allow driver better cope with lost bytes and prevent
spurious left/right button events when serial communication is
disturbed by a CPU-hungry real-time process.

Also fix some formatting.

Signed-off-by: Peter Osterlund <petero2@telia.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>