mfd: pcf50633-adc: Fix potential memleak in pcf50633_adc_async_read()

`req` is allocated in pcf50633_adc_async_read(), but
adc_enqueue_request() could fail to insert the `req` into queue.
We need to check the return value and free it in the case of failure.

Fixes: 08c3e06a ("mfd: PCF50633 adc driver")
Signed-off-by: Qiheng Lin <linqiheng@huawei.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20221208061555.8776-1-linqiheng@huawei.com