ath11k: fix missing error check on call to ath11k_pci_get_user_msi_assignment

The return error check on the call to ath11k_pci_get_user_msi_assignment is
missing.  If an error does occur, num_vectors is still set to zero and
later on a division by zero can occur when variable vector is being
calculated.  Fix this by adding an error check after the call.

Addresses-Coverity: ("Division or modulo by zero")
Fixes: d4ecb90b

 ("ath11k: enable DP interrupt setup for QCA6390")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200819111452.52419-1-colin.king@canonical.com