drivers: regulator: shell: fix isdigit() usage

ISO/IEC 9899:1999 (C Standard), §7.4 Character handling <ctype.h>:

In all cases the argument is an int, the value of which shall be
representable as an unsigned char or shall equal the value of the macro
EOF. If the argument has any other value, the behavior is undefined.

So add a cast to unsigned char to make sure we do not trigger UB.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>