drivers: mcr20a: cleanup and refactor interrupt processing

The interrupt processing of MCR20A was flawed and complicated.
This patch simplifies the handling of interrupts and reduces
the number of necessary SPI transfers.

Minor fixes:
 - use mutex for the PHY access control
 - remove unnecessary mcr20a_mask_irqb calls
 - do not read RX_FRM_LEN twice
 - increase timeout for sequence synchronization semaphore
   if the log level greater than 1
 - enable only the Sequence-end (SEQIRQ) interrupt
 - fix magic in NET_DEVICE_INIT
 - make the timeout values dependent on the log level

Change-Id: Ib3f64a092ffba91c80ff6e1d5cec995ab9d40bfb
Signed-off-by: Johann Fischer <j.fischer@phytec.de>