slimbus: qcom: remove unused variable

This patch fixes below warning when driver is compiled with W=1
qcom-ctrl.c: In function 'qcom_slim_rxwq':
qcom-ctrl.c:442:13: warning: variable 'len' set but not used
 [-Wunused-but-set-variable]

len seems to be unused in this function, so remove it.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>