drivers: wifi: esp: fix AT+CWMODE_CUR compatibility with ESP AT 1.7

There was a regression when implementing automatic AT+CWMODE{,_CUR}
handling based on driver needs. ESP AT 1.7 firmware does not support
AT+CWMODE_CUR=0, which means that radio needs to be either in STA, AP or
STA+AP mode (no NONE state available).

Fix ESP AT 1.7 compatibility by keeping radio in STA mode whenever it is
not used.

Move also first AT+CWMODE_CUR invocation before AT+CWDHCP_CUR, so that
the latter executes successfully with ESP AT 1.7.

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