lib/string_helpers: Introduce string_upper() and string_lower() helpers

[ Upstream commit 58eeba0bdb52afe5c18ce2a760ca9fe2901943e9 ]

Provide the helpers for string conversions to upper and lower cases.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Stable-dep-of: 3c0f4f09c063 ("usb: gadget: u_ether: Fix host MAC address case")
Signed-off-by: Sasha Levin <sashal@kernel.org>