powerpc/44x: Fix a spelling mismach to mismatch in head_44x.S

s/mismach/mismatch/

Signed-off-by: Bhaskar Chowdhury <unixbhaskar@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210202093746.5198-1-unixbhaskar@gmail.com