Move to default_instance QSPIF

[pennam]

After discussion on #13.

If WiFi interface is not initialized *qspi_bd is uninizialide too and is not possible to use it. Switching to mbed::BlockDevice::get_default_instance(); ensures to always get a valid block device instance.

[github-actions]

Memory usage change @ 7f37a53

Board	flash	%	RAM for global variables	%
arduino:mbed:envie_m7	🔺 0 - +24	0.0 - 0.0	0 - 0	0.0 - 0.0

Click for full report table

Board	examples/OTA_Qspi_Flash flash	%	examples/OTA_Qspi_Flash RAM for global variables	%	examples/OTA_SD_Portenta flash	%	examples/OTA_SD_Portenta RAM for global variables	%	examples/OTA_Usage_Portenta flash	%	examples/OTA_Usage_Portenta RAM for global variables	%
arduino:mbed:envie_m7	24	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0

Click for full report CSV

Board,examples/OTA_Qspi_Flash<br>flash,%,examples/OTA_Qspi_Flash<br>RAM for global variables,%,examples/OTA_SD_Portenta<br>flash,%,examples/OTA_SD_Portenta<br>RAM for global variables,%,examples/OTA_Usage_Portenta<br>flash,%,examples/OTA_Usage_Portenta<br>RAM for global variables,%
arduino:mbed:envie_m7,24,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0