powerpc/32s: mfsrin()/mtsrin() become mfsr()/mtsr()

powerpc/32s: mfsrin()/mtsrin() become mfsr()/mtsr()

Function names should tell what the function does, not how.

mfsrin() and mtsrin() are read/writing segment registers.

They are called that way because they are using mfsrin and mtsrin
instructions, but it doesn't matter for the caller.

In preparation of following patch, change their name to mfsr() and mtsr()
in order to make it obvious they manipulate segment registers without
messing up with how they do it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/f92d99f4349391b77766745900231aa880a0efb5.1612612022.git.christophe.leroy@csgroup.eu