qtnfmac: fix error handling in control path

This patch fixes the following warnings:

- smatch
drivers/net/wireless/quantenna/qtnfmac/commands.c:132 qtnf_cmd_send_with_reply() warn: variable dereferenced before check 'resp' (see line 117)
drivers/net/wireless/quantenna/qtnfmac/commands.c:716  qtnf_cmd_get_sta_info() error: uninitialized symbol 'var_resp_len'.
drivers/net/wireless/quantenna/qtnfmac/commands.c:1668 qtnf_cmd_get_mac_info() error: uninitialized symbol 'var_data_len'.
drivers/net/wireless/quantenna/qtnfmac/commands.c:1697 qtnf_cmd_get_hw_info() error: uninitialized symbol 'info_len'.
drivers/net/wireless/quantenna/qtnfmac/commands.c:1753 qtnf_cmd_band_info_get() error: uninitialized symbol 'info_len'.
drivers/net/wireless/quantenna/qtnfmac/commands.c:1782 qtnf_cmd_send_get_phy_params() error: uninitialized symbol 'response_size'.
drivers/net/wireless/quantenna/qtnfmac/commands.c:2438 qtnf_cmd_get_chan_stats() error: uninitialized symbol 'var_data_len'.

- gcc-8.2.1
drivers/net/wireless/quantenna/qtnfmac/commands.c: In function 'qtnf_cmd_send_with_reply':
drivers/net/wireless/quantenna/qtnfmac/commands.c:133:54: error: 'resp' may be used uninitialized in this function [-Werror=maybe-uninitialized]

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>