tools: ynl-gen-c: Fix remaining pylint warnings

tools: ynl-gen-c: Fix remaining pylint warnings

Fix the following pylint warning instances:

ynl_gen_c.py:575:15: E0606: Possibly using variable 'mem' before
assignment (possibly-used-before-assignment)

ynl_gen_c.py:888:0: R1707: Disallow trailing comma tuple
(trailing-comma-tuple)

ynl_gen_c.py:944:21: C0209: Formatting a regular string which could be an
f-string (consider-using-f-string)

ynl_gen_c.py:1450:14: C1802: Do not use `len(SEQUENCE)` without comparison
to determine if a sequence is empty (use-implicit-booleaness-not-len)

ynl_gen_c.py:1688:13: W1514: Using open without explicitly specifying an
encoding (unspecified-encoding)

ynl_gen_c.py:3446:0: C0325: Unnecessary parens after '=' keyword
(superfluous-parens)

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20260108161339.29166-14-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>