drivers: wifi: esp: improve readability of CWMODE changes

AT+CWMODE command controls in which mode (NONE, STA, AP, STA+AP) ESP
chip is operating. Add helper macros to replace usage of magic
numbers (0-3), hence improve readability. Add also a esp_mode_switch()
helper function, which allows to conveniently switch to chosen mode.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>