LAN91C111: Add functions to save/restore context during ISR

The selected register bank and content of the pointer register
need to be preserved during an ISR as they can change depending on the
operation performed in the ISR.

The commit also include changes to improve readability of the
select_registers_bank() function.

Signed-off-by: Hugues Kamba-Mpiana <hugues.kambampiana@arm.com>