powerpc/time: Avoid using get_tbl()

get_tbl() is confusing as it returns the content TBL register
on PPC32 but the concatenation of TBL and TBU on PPC64.

Use mftb() instead.

This will allow the removal of get_tbl() in a following patch.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/decefb47c8a2070bf55d20b096b813908c7b3110.1612866360.git.christophe.leroy@csgroup.eu