The assignment operation after a & mask operation is redundant, the
variables being assigned are not used afterwards. Replace the &=
operator with just & operator.

Cleans up two clang-scan warnings:
drivers/clk/socfpga/clk-gate.c:37:10: warning: Although the value stored
to 'l4_src' is used in the enclosing expression, the value is never
actually read from 'l4_src' [deadcode.DeadStores]
                return l4_src &= 0x1;
                       ^         ~~~
drivers/clk/socfpga/clk-gate.c:46:10: warning: Although the value stored
to 'perpll_src' is used in the enclosing expression, the value is never
actually read from 'perpll_src' [deadcode.DeadStores]
                return perpll_src &= 0x3;
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Link: https://lore.kernel.org/r/20211230150321.167576-1-colin.i.king@gmail.comAcked-by: Dinh Nguyen <dinguyen@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>