drivers: mcr20a: small improvements

Issue a error message, if the _mcr20a_read_reg fails.

Do not execute SPI burst read/write if the buffer is too small.

Unlock mutex if set_pan_id, set_short_addr or set_ieee_addr
fail.

Force abort of the sequence when the higher level changes the channel
even though a T or TR sequence is in progress.

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