ath11k: Remove redundant assignment to variable fw_size

Variable fw_size is being assigned a value that is never read and
being re-assigned a new value in the next statement. The assignment
is redundant and can be removed.

Addresses-Coverity: ("Unused value")
Fixes: 336e7b53 ("ath11k: clean up BDF download functions")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211006105529.1011239-1-colin.king@canonical.com