wilc1000: add valid vmm_entry check before fetching from TX queue

'vmm_table' array contains the size of data buffer length including host
header length. In 'vmm_table' array, the Zero value means the end of
vmm_entries that needs to transfer to firmware which is calculated based on
VMM free size in firmware.

Use 'vmm_table' valid entry check before fetching the entry from TX queue to
only copy valid number of entries to avoid possible NULL pointer exception
observed sometimes during large file transfers.

Signed-off-by: Ajay Singh <ajay.kathat@microchip.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220504161924.2146601-5-ajay.kathat@microchip.com