fix(spm): silence warning in sp_mk_generator

Silence warning from sp_mk_generator that 'is not' operator
is not meant for integers. This replaces the referred instance
with '!='.

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: I0d31ad65466dbeafebbfc929e506c3e290913aca
1 file changed