Use sed instead of grep+cut to capture the passphrase

[warpdesign]

Current method would crop any passphrase containing an = sign.

For eg. with this hostapd.conf:

...
wpa_passwphrase=Foo=Bar
...

the wpa_passphrase() function would would return Foo instead of Foo=Bar, incorrectly cropping the
passkey.

As a result, when editing the hotspot settings, the user may incorrectly update its passkey without even knowing it.
Luckily, mine was cropped to a too short string, so I noticed the error in the form.

I tested the new command with macOS & Linux (Debian) so I guess it should work with most Unix OS.

Also, I guess the problem can happen in other places, like maybe with the ssid (can it contain = signs?).

[billz]

@NL-TCH

[NL-TCH]

good catch @warpdesign, thanks for the PR!

@billz good to merge from my side

[NL-TCH]

we might need to update other variables as well, now the "=" sign is nowhere allowed (for example in SSID-names and Interfaces))