efi: clocksoure: make sure the meassured frequency is valid
Some EFI implementations have a even slower tick rate and the meassured
frequency may be zero.
Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>