lib: utils: Simplify SET_ISA_EXT_MAP()

The define is hard to read. The continue statement does not do what was
intended.

* Remove do {} while (false);
* Change the name to set_multi_letter_ext
  - Other local macros are lower case too.
  - Refer to the fact that this is only used for multi-letter extensions.

Addresses-Coverity-ID: 1568359 Unexpected control flow
Fixes: d72f5f17

 ("lib: utils: Add detection of Smepmp from ISA string in FDT")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>