staging: rtl8192u: check return value of read_nic_word_E

The call of read_nic_word_E may fail, therefore its return value must be
checked and propagated in the case of error.

Signed-off-by: Salah Triki <salah.triki@acm.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>