drivers: wifi: esp: fix hostname configuration

By the time hostname configuration was implemented, driver was switching
only between STA and STA+AP modes. After dynamic selection between NONE,
STA, AP and STA+AP was implemented (commit referenced below), hostname
configuration no longer takes effect when ESP chip obtains address over
DHCP (and sends hostname in the DHCP request).

Set hostname each time after enabling STA mode, so that it takes effect
in DHCP requests.

Fixes: 03ce6100 ("drivers: wifi: esp: control CWMODE depending on
  current needs")
Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>