Backport ARMmbed#12038: Remove qspi_frequency() call.

cy_qspi_frequency is not implemented.
This change is made because for compatibility
reason with upcoming cy_hal changes.

targets/TARGET_Cypress/TARGET_PSOC6/cy_qspi_api.c

@@ -38,7 +38,8 @@ qspi_status_t qspi_free(qspi_t *obj)
 
 qspi_status_t qspi_frequency(qspi_t *obj, int hz)
 {
-    return CY_RSLT_SUCCESS == cyhal_qspi_set_frequency(&(obj->hal_qspi), (uint32_t)hz) ? QSPI_STATUS_OK : QSPI_STATUS_ERROR;
+    /* Return OK since this API is not implemented in cy_hal */
+    return QSPI_STATUS_OK;
 }
 
 static inline cyhal_qspi_bus_width_t cyhal_qspi_convert_width(qspi_bus_width_t width)