clk: actions: remove redundant assignment after a mask operation

The assignment operation after a & mask operation is redundant,
the &= operator can be replaced with just the & operator.

Cleans up a clang-scan warning:
drivers/clk/actions/owl-pll.c:28:9: warning: Although the value
stored to 'mul' is used in the enclosing expression, the value is
never actually read from 'mul' [deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Link: https://lore.kernel.org/r/20220418141537.83994-1-colin.i.king@gmail.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>