mpt2sas: cleanup trivial smatch complaints

Fix smatch complains about buffer size in _ctl_do_mpt_command where
(uint32_t) bytes are copied into a buffer sized in (u16). The function
verifies that the number of bytes will safely fit inside the buffer,
however modifying the code is trivial to avoid the message.

Smatch also suggests a few return values in mpt2sas_scsih.c. The changes
are small and the callers don't care, they only check against non-zero.

Signed-off-by: Joe Lawrence <joe.lawrence@stratus.com>