clk: stm32h7: Remove an unused field in struct stm32_fractional_divider

'mmask' has never been used in this driver.
Remove it.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/e08a470fbd6151ebd83a548714c08807a80a8ad0.1680364296.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Stephen Boyd <sboyd@kernel.org>