spmi: pmic-arb: Remove checking opc value not less than 0

The opc parameter in pmic_arb_write_cmd() function is defined with type
u8 and it's always greater than or equal to 0. Checking that it's not
less than 0 is redundant and it can cause a forbidden warning during
compilation. Remove the check.

Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>