MFC r261491 (by ambrisko):
Add a tunable "hw.mfi.mrsas_enable" to allow mfi(4) to drop priority and
allow mrsas(4) from LSI to attach to newer LSI cards that are support by
mrsas(4).  If mrsas(4) is not loaded into the system at boot then mfi(4)
will always attach.  If a modified mrsas(4) is loaded in the system.  That
modification is return "-30" in it's probe since that is between
BUS_PROBE_DEFAULT and BUS_PROBE_LOW_PRIORITY.

This option is controller by a new probe flag "MFI_FLAGS_MRSAS" in mfi_ident
that denotes cards that should work with mrsas(4).  New entries that should
have this option.

This is the first step to get mrsas(4) checked into FreeBSD and to avoid
collision with people that use mrsas(4) from LSI.  Since mfi(4) takes
priority, then mrsas(4) users need to rebuild GENERIC.  Using the
.disabled="1" method doesn't work since that blocks attaching and the
probe gave it to mfi(4).

MFC r267451 (by delphij):
Correct variable for loader tunable variable hw.mfi.mrsas_enable.