ath11k: return error if firmware request fails

In ath11k_qmi_prepare_bdf_download(), ath11k_core_firmware_request() is
called, but the returned pointer is not checked for errors. Rather the
variable ret (which will always be zero) is checked by mistake. Fix
this and replace the various gotos with simple returns for clarity.

While we are at it, move the call to memset, as variable bd is not used
on all code paths.

Fixes: 7b57b2dd

 ("ath11k: create a common function to request all firmware files")
Signed-off-by: Alex Dewar <alex.dewar90@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200825143040.233619-1-alex.dewar90@gmail.com