LinuxKPI: add get_unaligned_le16()
Add get_unaligned_le16() to asm/unaligned.h needed by a driver.
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D34224
(cherry picked from commit 2e818fbcfc03f9416593d5fe5bc860c1829f5366)