Moved the READ and WRITE switch cases to the top. Added a  default
case to the switch case and replaced duplicate scsi result value with a
macro.

The idea is that since most of scsi commands we care about performance
wise are read or write, we need to process them first.

Internally the compiler (GCC) converts a switch case into either a jump
table or a bunch of if else conditions, so placing the often used read,
write cases at the top is an effort in optimization.
Signed-off-by: NRaghava Aditya Renukunta <RaghavaAditya.Renukunta@microsemi.com>
Signed-off-by: NDave Carroll <David.Carroll@microsemi.com>
Reviewed-by: NJohannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>