mpt3sas: Fix static analyzer(coverity) tool identified defects

1.Wrong size of argument is being passed
 The size of struct being passed as an argument to memset func and area of
 memory being pointed by an instance of struct in memset func should be of
 same structure type.
2.Dereference null return value
3.Array compared against '0'
 Check whether value pointed by particular index of an array is null or not
 in "if" statement.

Signed-off-by: Suganath prabu Subramani <suganath-prabu.subramani@avagotech.com>
Signed-off-by: Chaitra P B <chaitra.basappa@avagotech.com>
Reviewed-by: Tomas Henzl <thenzl@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>