rsxx: Remove unnecessary parentheses

Clang warns when more than one set of parentheses is used for a
single conditional statement:

drivers/block/rsxx/cregs.c:279:15: warning: equality comparison with
extraneous parentheses [-Wparentheses-equality]
        if ((cmd->op == CREG_OP_READ)) {
             ~~~~~~~~^~~~~~~~~~~~~~~
drivers/block/rsxx/cregs.c:279:15: note: remove extraneous parentheses
around the comparison to silence this warning
        if ((cmd->op == CREG_OP_READ)) {
            ~        ^              ~
drivers/block/rsxx/cregs.c:279:15: note: use '=' to turn this equality
comparison into an assignment
        if ((cmd->op == CREG_OP_READ)) {
                     ^~
                     =
1 warning generated.

Reported-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>