Fix issues reported with internal sd-card

[gurcei]

Test Environment (required) As provided by Yuriksha and MickOz

• Platform: MEGA65r3
• Core Commit: 93d55f08 (tag: release-0.95)
• ROM Release: (i.e. 920377)

Describe the bug Several users have recently reported similar issues relating to problems getting mega65-config and/or mega65-fdisk working properly:

• yuriksha
• mickoz
• remz
• MattJD
• Rob Colton

Full details of past debugging efforts in this lengthy discord thread

To Reproduce

Not all users are impacted by this issue, but it appears a subset of our user-base is (so potentially could relate to borderline behaviour in some of the sd-cards, as yuriksha tried a different sd-card in the internal slot and met with success - see this thread). I believe the steps to replicate are:

1. Do an ALT+power on
2. Go to the mega-config menu
3. Get greeted by a message saying there was a problem in the detection of your sd card.

Expected behavior Should successfully enter mega65-config, correctly detecting the sd-card.

[gurcei]

Ok, I've rebased my efforts onto the development branch, will make a PR soon

[gurcei]

My commit was merged into development branch