Change normal increment to postfix

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Add new stats members (SPI errors, write buffer misses)

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Implement new stats counter (SPI errors, write buffer misses)

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Add new stats members to DebugFS (SPI errors, write buffer misses)

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Add NULL checks

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reduce variable types to it's needs

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>